KR20010040801A - Device for optically scanning a record carrier - Google Patents

Abstract

A device is described for optically scanning two types of record carrier (1; 25) with a radiation beam (15) having a high numerical aperture. The radiation beam is focused through a transparent layer (2; 26) of the record carrier on an information layer (3; 27) by means of an objective lens (10) and a plano-convex lens (11). The magnifying power of the plano-convex lens is equal to 1/n1, i.e. the inverse of the refractive index of the transparent layer (2). The gap between the plano-convex lens and the record carrier can be maintained at a fixed value when the scanning is changed between the first and the second record carrier.

Description

TECHNICAL FIELD [0001] The present invention relates to an optical scanning apparatus for a recording medium,

The present invention relates to an optical scanning apparatus for optically scanning recording media of first and second types, wherein the first type of recording medium has a first information layer and a first transparent layer having a first thickness, The second type recording medium has a second information layer and a second transparent layer having a refractive index n ₁ and a second thickness larger than the first thickness, the optical scanning device comprising an objective lens and a first or second transparent layer And a convex lens for converging the radiation beam to the focus on the information layer via the objective lens, wherein the convex surface facing the objective lens, the plane opposed to the substantially transparent layer, the convex lens having the magnification and the refractive index n ₂ are axially positioned .

The amount of information that can be stored in the optical recording medium is particularly dependent on the size of the radiation spot formed by the scanning device on the information layer of the recording medium. By reducing the size of the spot, the information density and thus the amount of stored information can be increased. The spot size at this time can be reduced by increasing the numerical aperture of the radiation beam forming the spot. When using a single objective lens, such an increase in the numerical aperture generally involves a reduction in the free working distance of the lens forming the radiation beam, i.e. the minimum distance between the recording medium and the lens. The cost of manufacturing these objectives is increased at higher numerical apertures, the field of view of this lens is reduced, and this lens increases the problem of material dispersion. These problems may be mitigated by inserting a convex lens between the objective lens and the recording medium. This convex lens may be plano-convex. The convex lens may also be a slider lens or a solid immersion lens and may be arranged at a very small distance on the recording medium by means of determining its axial position, i.e. its position along the optical axis of the lens have. The entire convergence speed of the radiation beam is distributed to the objective lens and the plano convex lens. The advantage of using this flat convex lens is that it adds little aberration to its radiation beam.

A scanning device having such a convex lens is known from European Patent Application No. 0 727 777. This apparatus has an optical head for converging a radiation beam with numerical aperture (NA) of 0.84 so that an objective lens and a planoconvex lens scan a recording medium through a transparent layer. And aligns the flat convex lens at a small height above the recording medium. The lens at this time may be mounted on a slider that slides on the recording medium or floats on a thin air layer. The lens also has a free working distance of a few micrometers. This known apparatus is disadvantageous in that the free working distance thereof will be greatly reduced when the scanning is changed to a recording medium in which the transparent layer has a larger thickness. In some embodiments, the free working distance changes from 300 to 125 占 퐉 when the thickness of the transparent layer is changed from 100 to 600 占 퐉. This reduction requires the height of a plano convex lens that can be adjusted according to the type of recording medium to be scanned. This also affects the action of the servo controlling the dynamic action of the plano-convex lens, particularly the axial position and / or the inclination of the lens, and increases the risk of collision between the optical head and the recording medium.

It is an object of the present invention to provide a scanning apparatus for scanning two types of recording media with a high numerical aperture radiation beam to solve the above disadvantage.

The present invention achieves the above object by means of a scanning device as described in the opening paragraph, and the magnification of the convex lens is substantially equal to 1 / n ₁ , and the means is to keep the distance between the convex lens and the recording medium at a predetermined value . This scanning apparatus has an advantage that the distance between the convex lens and the recording medium has almost the same value for the recording medium of the first and second types. Thus, since the same suspension of plano convex lenses can be used for both types of recording media, it is possible to simplify the design of the scanning device and to improve the tolerance of the optical head of the scanning device colliding into the recording medium have. The servo system of the scanning device for positioning the objective lens and / or the convex lens in the axial direction is relatively simple. Such a servo system must adjust the position of the objective lens so that the radiation beam is focused on the information layer and the distance between the convex lens and the recording medium is maintained at a constant value. At this time, the servo system does not require different settings for different recording media. In the scanning apparatus according to the present invention, the spherical aberration compensation is automatically adjusted while the recording medium is being scanned.

The magnification is preferably 1 / n ₁ when scanning the form of the recording medium which requires a very large numerical aperture (NA). Generally, this is in the form of a recording medium having a very thin transparent layer. When the refractive index of the transparent layer is in the range between 1.5 and 1.6, the size is between 0.625 and 0.667, i.e., 0.646 0.021.

The present invention also relates to an optical scanning apparatus for scanning an optically recording medium having at least two information layers and a transparent spacer layer arranged between information layers with a refractive index n ₁ , A convex lens for converging the beam to a focus on one of the information layers, and a convex lens for axially positioning a convex lens facing the objective lens, a substantially planar surface opposite to the transparent layer, a magnification and a refractive index n ₂ Wherein the magnification of the convex lens is substantially equal to 1 / n _1, and the means is to maintain the distance between the convex lens and the recording medium at a predetermined value. When the scanning is changed from one information layer to another information layer, the axial position of the objective lens is changed so that the focus of the radiation beam moves to the desired information layer, while the distance between the convex lens and the incident surface of the recording medium, There is no need to change the 'gap'. The distance between the convex lens and the incident surface of the recording medium can be maintained at a fixed value, thereby simplifying the configuration of the optical head of the scanning apparatus.

The refractive index n ₁ of the transparent layer is preferably larger than the refractive index n ₂ of the convex lens. The spherical aberration caused by the gap between the convex lens having a refractive index smaller than n ₂ and the transparent layer having a refractive index larger than n ₂ compensates at least partially with each other. Thus, the amount of spherical aberration induced by the objective lens is reduced, which in turn further increases the positioning tolerance. The refractive index at this time is preferably related to (n ₁ -1)> 1.03 (n ₂ -1).

To obtain the minimum spherical aberration at the focus position, the gap size d _gap preferably depends on the refractive index n ₂ of the convex lens and n ₁ of the transparent substrate according to the following relationship:

d _gap / d _s = (n ₂ / n ₁ ³ ) * (n ₁ ² -n ₂ ² ) / (n ₂ ² -1). The value of d _gap is preferably within the range of 40% for a value of d _s larger than d _gap , where d _s is the thickness of the transparent layer.

The value of d _s is preferably greater than the value of d _gap to obtain the refractive index of the usable materials.

The product of the magnification and the refractive index n ₁ is preferably in the range of 0.95 to 1.05.

The objects, advantages and features of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention, as illustrated in the accompanying drawings.

Figure 1 shows a scanning device,

Fig. 2 shows an objective lens and a convex lens,

3 shows three different types of recording media,

Fig. 4 shows an embodiment of an objective lens and a plano-concave lens.

Fig. 1 shows an apparatus for scanning an optical recording medium 1. Fig. This recording medium has a transparent layer 2 on one side where the information layer 3 is formed. The side surface of the information layer, which is opposed to the transparent layer, is protected from environmental pollution by the protective layer 4. The side surface of the transparent layer opposed to the device is referred to as an incident surface 5. This transparent layer 2 acts as a support for a recording medium by providing a mechanical support for the information layer. This transparent support has a unique function for protecting the information layer, which mechanical support is provided on the other side of the information layer, for example the information layer 4 or more, and the information layer 3 Or more transparent layers connected thereto. The information is stored in the information layer 3 of the recording medium in the form of optically detectable marks arranged in approximately parallel, concentric or spiral tracks not shown in the figure. These marks may be in any optically readable form, for example in the form of pits, or in the form of regions having a reflection coefficient or a magnetization direction different from their surroundings, or a combination of these types.

This scanning device has, for example, a semiconductor laser, a radiation source 6 for emitting a radiation beam 7 to be emitted. The beam splitter 8, for example a translucent plate, reflects radiation towards the lens system. This lens system has a collimator lens 9, an objective lens 10 and a convex lens 11. [ At this time, the collimating lens 9 changes the diverging radiation beam 7 into the collimated beam 12. [ The objective lens 10 having an optical axis 13 converts the collimated radiation beam 12 into a converging beam 14 that is incident on the lens 11. At this time, the collimator lens 9 and the objective lens 10 may be combined into a single lens. The convex lens 11 converts the incident beam 14 into a converging beam 15 to become a focus 16 on the information layer 3. [ The convex lens 11 has a substantially flat surface with respect to the convex surface, that is, the lens 11 is a flat convex lens. This plane may have an aspheric profile without significant light intensity. This plane may be small enough so that its curvature does not significantly affect the optical performance of the convex lens, for example, for aerodynamic purposes. Further, this plane forms a gap between the lens and the layer, facing the transparent layer 2. Although the objective lens 10 is shown in the figure as a single element, the objective lens 10 may comprise more elements and may also include a hologram that operates upon transmission or reflection, or radiation from a waveguide that transmits the radiation beam And may have a grating for coupling. The radiation of the converging beam 15 reflected by the information layer 3 forms a reflected beam 17 and returns to the optical path of the forward converging beam 14. At this time, the objective lens 10 and the collimator lens 9 convert the reflected beam 17 into a convergent reflected beam 18, and the beam splitter 8 converts the reflected beam 17 toward the detection system 19 18 to separate the forward direction and the reflected beam. The detection system detects the radiation and converts it into one or more electrical signals. One of these signals is a value indicating information read from the information layer 3. The other signal is the focus error signal 21 indicating the difference in axial height between the focus 16 and the information layer 3. This focus error signal is used as an input for the focus servo controller 22 which controls the axial position of the objective lens 10 and / or the convex lens 11, thereby controlling the axial position of the focus 16 Substantially coincides with the plane of the information layer 3. The portion of the detection system including the detection elements having sensitivity to one or more radiation and the electronic circuit for processing the output signals of the detection elements is used to generate a focus error called a focus error detection system. The focus servo system for determining the position of the lens system includes a focus error detection system, a focus servo controller, and an actuator for driving the lens system.

The gap, that is, the distance between the plane of the lens 11 and the incident surface 5 of the recording medium 1, should be kept at a very small value regardless of the form of the recording medium and the position of the information layer in the recording medium . This can be achieved by using the passive air bearing structure transfer lens 11 and can be designed to maintain a very small value of gap. It is also possible to use an optical drive error signal that represents the deviation of the actual gap from a very small value of the gap so that the particular actuator uses the error signal as its input signal for the actuator servo loop, Keep the above distance. The actuator of the lens 10 is controlled by the focus error signal 21 in order to keep the focus 16 on the information layer 3.

The spherical aberration that occurs when the radiation beam is focused through a transparent layer that is thicker than the design thickness of the information layer is compensated by the focusing operation of the objective lens. At this time, the focusing operation causes the amount of spherical aberration to remove the aberration produced by the thicker transparent layer due to the change in the size of the flat convex lens. During the focusing operation of the objective lens, the gap must be maintained at a very small value of the objective lens in the manner mentioned above. The change in the thickness of the transparent layer of the recording medium is generally of a small percentage, for example a thickness of less than 10 [mu] m for very few layers of 100 [mu] m thickness. For special applications, the change is within 3 μm p-p.

2 is an enlarged view of the objective lens 10 and the planoconvex lens 11. The objective lens 10 may be a single aspherical surface convex lens or a bi-aspherical surface lens. This objective lens 10 is also designed in a known manner to compensate for the spherical aberration induced by the convex lens 11 and the transparent layer 2, To form a beam.

The permissible error of the optical head for changing the thickness of the transparent layer 2 is approximately equal to the magnification B of the flat convex lens 11 of 1 / n ₁ , that is, the refractive index of the transparent layer. This can be seen from the following. When the distance between the lens 10 and the lens 11 changes, the conjugate distance of the lens 11 changes. When the image conjugate of the lens 11 changes by the amount of? 1, the spherical aberration W of the radiation beam 15 becomes

.

The spherical aberration caused by the thickness variation? D ₁ of the transparent layer 2 in the radiation beam 15 is the same as the following equation.

A conjugate of the lens 11 is changed by Δl if vary the thickness of the transparent layer by Δd _1, the distance between the incident surface of the lens 11 and the recording medium, d ₂ is Δd so that the radiation beam is focused on the information layer, _{2 < / RTI} >

to be.

The spherical aberration caused by the thickness change can be compensated by selecting? W ₁ = -ΔW ₂ , and as a result, the value of?

.

This is in accordance with the plane change of the _gap distance d between the incident surface 5 of the plane and the transparent layer (2) of the positive lens (11), Δd _gap,

to be. The term between square brackets can be made zero by taking the B value equal to 1 / n ₁ . In such a case changes in the distance Δd d _gap the _gap is within the approximation just not related to the change Δd in the thickness of the transparent layer ₁ (2). In other words, the free working distance of the lens 11 is independent of the thickness of the transparent layer.

This scanning device is designed to scan two different types of recording media. Figure 3 shows a first form of recording medium 25 with a transparent layer 26 having an incident surface 29 into which the radiation of the scanning device enters the recording medium. The recording medium 25 also has an information layer 27 and a protective layer 28. [ The recording medium of the second embodiment is the recording medium 1 shown in Fig. 1 and has a transparent layer 2, an information layer 3, and a protective layer 4. The information layers 3 and 27 may have different or similar information densities and the information may be stored in marks of different or similar forms in different or similar formats. The thickness of the transparent layer 2 is larger than the thickness of the transparent layer 26. The thickness of the transparent layer 26 is a zero value so that the recording medium is made of the so-called air-incident recording medium 30 including the information layer 31 and the protective layer or the support 32, as shown in Fig. In a specific example, the transparent layer 26 is a polycarbonate foil having a thickness of 100 탆, the transparent layer 2 is a polycarbonate layer having a thickness of 600 탆, and the information density of the information layer 27 is higher than that of the information layer 3. The gap between the convex lens 11 and the incident surface 5 is small enough to realize the required high numerical aperture of the radiation beam 15 and is small enough to cause the collision of the planar convex lens 11 or its holder with respect to the recording medium Is selected to be large enough to avoid.

When the scanning is changed from the recording medium 25 of the first embodiment to the recording medium 1 of the second embodiment, the convex lens 11 is located at the same height above the incident surface 5 as above the incident surface 29 . The axial position of the objective lens 10 is controlled by the focus servo system so that the focus 16 coincides with the information layer 27. The positioning of the two lenses 10 and 11 changes the distance between them in such a way that the lenses compensate for the change in spherical aberration caused by the thickness variation of the transparent layer. The dynamic action of the optical head will be very similar to the two types of recording media, since the gap between the plane convex lens 11 and the incidence plane of the two types of recording medium is almost the same.

Fig. 4 shows the objective lens 45 and the convex lens 46 for the scanning device. The wavelength of the radiation is 650 nm. The numerical aperture for scanning the recording medium 25 having a 100 占 퐉 polycarbonate transparent layer is 0.85. The numerical aperture for scanning the recording medium 1 having the 600 占 퐉 polycarbonate transparent layer is 0.60. The refractive index of polycarbonate at 650 nm wavelength is 1.5806. This plano convex lens 46 is made of Schott glass FK3 having a refractive index of 1.46 and one plane. The radius of curvature of the convex surface of the flat convex lens 46 at this time is 1.25 mm, and the thickness of the lens on the optical axis is 1.25 mm. The value of n ₁ B for the lens 11 is 0.985. The distance between the plane 47 of the recording medium 25 and the incident surface 29 is designed to be 100 mu m. When the scanning is changed to the recording medium 1, the distance between the plane and the incident surface 5 is changed to 93 mu m, and optimum spherical aberration compensation, that is, a change of 7 mu m on the 100 mu m distance is obtained. Therefore, maintaining an air gap equal to 100 mu m will be an effective spherical aberration compensation when changing from the recording medium of the first type to the recording medium of the second type.

The objective lens 45 is made of a short glass LAKN22 having a refractive index of 1.648. The convex surface 48 of the objective lens has a curvature radius r ₁ of 2.429 mm and the concave surface 49 has a curvature radius r ₂ of 180.368 mm. The thickness of the lens on the optical axis is 1.9 mm. The distance on the optical axis between the lenses 45 and 46 is 0.010 when scanning through the 600 mu m transparent layer of the recording medium 1 and 0.985 mm when scanning through the 100 mu m transparent layer of the recording medium 25. [ The two surfaces 48 and 49 have the aspheric profile described by the following equation.

Here, when c ₁ = 1 / r ₁ for surface 48 and c ₂ = 1 / r ₂ for surface 49, z is the coordinate along the optical axis and r is the radial coordinate to be. The values of the coefficients a ₂ to a ₁₆ are as follows.

The present scanning device is also very suitable for scanning the multilayer recording medium 35 as shown in Fig. Such a recording medium has a transparent layer 36 having an incident surface 37, two information layers 38 and 39 separated by a transparent spacer layer 40, and a protective cover layer 41. At this time, the spacer layer may be a thin layer attached between the information layers or a UV cured spin coating layer having a thickness of 30 mu m. The material of the spacer layer may be a polymer such as a polycarbonate having a refractive index of 1.58. When scanning is changed from the information layer 38 to the information layer 39, the objective lens 10 moves in the axial direction so that the focus 16 changes from the information layer 38 to the information layer 39. The distance between the flat convex lens 11 and the incident surface 37 of the recording medium does not change, that is, the free working distance remains substantially the same. A change in the distance between the objective lens 10 and the plano convex lens 11 causes spherical aberration to compensate for the spherical aberration caused by the spacer layer 40. [ The combination of the objective lens and the flat convex lens operates as an adjustable spherical aberration compensator.

Claims (6)

In order to optically scan the first and second types of recording media, the first type of recording medium has a first information layer and a first transparent layer having a first thickness, 2 information layer and a second transparent layer having a refractive index n ₁ and a second thickness greater than the first thickness, the objective lens comprising an objective lens and a second transparent layer for converging the radiation beam onto the focus of the information layer via the first or second transparent layer There is provided an optical scanning device having a convex lens and a means for axially positioning a convex lens facing the objective lens, a substantially planar surface facing the transparent layer, and a convex lens having a magnification and a refractive index n ₂ , Wherein magnification of the lens is substantially equal to 1 / n ₁ , and the means maintains the distance between the convex lens and the recording medium at a predetermined value. The method according to claim 1, Wherein the value of n ₁ is greater than the value of n ₂ . 3. The method of claim 2, Wherein the refractive indices n ₁ and n ₂ are related to (n ₁ -1)> 1.03 (n ₂ -1). In order to optically scan the recording medium, it is necessary to have at least two information layers and a transparent spacer layer arranged between the information layers with a refractive index n ₁ and to have the objective lens and the radiation beam focused at one of the information layers And a means for axially positioning a convex lens facing the objective lens, a substantially planar surface facing the transparent layer, and a convex lens having a magnification and a refractive index n ₂ , the optical scanning device comprising: Wherein magnification of the convex lens is substantially equal to 1 / n _1, and the means maintains the distance between the convex lens and the recording medium at a predetermined value. 5. The method of claim 4, Wherein the value of n ₁ is greater than the value of n ₂ . 6. The method of claim 5, Wherein the refractive indices n ₁ and n ₂ are related to (n ₁ -1)> 1.03 (n ₂ -1).